Radio Frequency Identification (RFID) transponders (tags) are operated in conjunction with RFID base stations for a variety of inventory-control, security and other purposes. Typically an item having a tag associated with it, for example, a container with a tag placed inside it, is brought into a "read zone" established by the base station. The RFID base station generates a continuous wave electromagnetic disturbance at a carrier frequency. This disturbance is modulated to correspond to data that is to be communicated via the disturbance. The modulated disturbance, which carries information and may be sometimes referred to as a signal, communicates this information at a rate, referred to as the data rate, which is lower than the carrier frequency. The transmitted disturbance will be referred to hereinafter as a signal or field. The RFID base station transmits an interrogating RF signal which is modulated by a receiving tag. The tag modulates the RF signal in order to impart to the signal information stored within the tag. The receiving tag then transmits the modulated, answering, RF signal to the base station.
RFID tags may be active, containing their own RF transmitter, or passive, having no transmitter. Passive tags, i.e., tags that rely upon modulated back-scattering to provide a return link to an interrogating base station, may include their own power sources, such as a batteries, or they may be "field-powered", whereby they obtain their operating power by rectifying an interrogating RF signal that is transmitted by a base station. Although both battery-powered and field powered tags have minimum RF field strength read requirements, or read thresholds, in general, a field-powered passive system requires at least an order of magnitude more power in the interrogating signal than a system that employs tags having their own power sources. Because the interrogating signal must provide power to a field-powered passive tag, the read threshold for a field-powered passive tag is typically substantially higher than for an active tag. However, because field-powered passive tags do not include their own power source, they may be substantially less expensive than active tags and because they have no battery to "run down", field-powered passive tags may be more reliable in the long term than active tags. And, finally, because they do not include a battery, field-powered passive tags are typically much more "environmentally-friendly".
Although field-powered passive tag RFID systems provide cost, reliability, and environmental benefits, there are obstacles to the efficient operation of field-powered passive tag RFID systems. In particular, it is often difficult to deliver sufficient power from a base station to a field-powered passive tag via an interrogating signal. The amount of power a base station may impart to a signal is limited by a number of factors, not the least of which is regulation by the Federal Communication Commission (FCC). Consequently, an RFID tag that is compact, light weight, low cost and that efficiently employs a substantial portion of the signal energy which it receives from an interrogating base station would be highly advantageous.
Related applications and issued patents
Related U.S. Patents assigned to the assignee of the present invention include: U.S. Pat. Nos. 5,528,222; 5,550,547; 5,552,778; 5,554,974; 5,538,803; 5,563,583; 5,565,847; 5,606,323; 5,521,601; 5,635,693; 5,673,037; 5,682,143; 5,680,106; 5,729,201; 5,729,697; 5,736,929; 5,739,754; and 5,767,789. Patent applications assigned to the assignee of the present invention include: application U.S. Pat. No. 5,673,037; No. 08/621,784, filed on Mar. 25, 1996 entitled, "Thin Radio Frequency Transponder with Leadframe Antenna Structure" by Brady et al. (pending); application No. 08/626,820, Filed: Apr. 3, 1996, entitled, "Method of Transporting RF Power to Energize Radio Frequency Transponders", by Heinrich et al.; application No. 08/694,606 filed Aug. 9, 1996 entitled, "RFID System with Write Broadcast Capability" by Heinrich et al.; application No. 08/681,741, filed Jul. 29, 1996 entitled, "RFID Transponder with Electronic Circuit Enabling and Disabling Capability", by Heinrich et al.; application No. 08/592,250 (See also PCT International Application No. PCT/EP95/03903 filed Sep. 20, 1995, and U.S. application No. 08/330,288 filed Oct. 27, 1994, now abandoned, on which the PCT application is based); U.S. Pat. No. 5,729,201; application No. 08/909,719; application No. 08/621,784; application No. 08/660,249; application No. 08/660,261; application No. 08/790,640; application No. 08/790,639; and application No. 08/681,742. The above identified U.S. Patents and U.S. Patent applications are hereby incorporated by reference. Additionally, Patent Applications entitled, "Radio Frequency Identification Transponder Having a Spiral Antenna", "Radio Frequency Identification Transponder Having a Helical Antenna", "RFID Transponder Having Improved RF Characteristics",and "Distributed Impedance Matching Circuit For High Reflection Coefficient Load", filed on the same day as this application and assigned to the same assignees as this application is assigned are also incorporated by reference.
The applicants claim priority under 35 U.S.C. 119 (e) for provisional applications having attorney docket numbers YO897 660P1, YO897-661P1, and YO997-038P1, respectively filed on March 16, 17, and 13.